The assignee of the present invention manufactures and deploys spacecraft for, commercial, defense and scientific missions. Many such spacecraft operate in a geosynchronous orbit having a period equal to one sidereal day (approximately 23.93 hours).
A particular type of geosynchronous orbit is a geostationary orbit (GSO), characterized as being substantially circular and co-planar with the Earth's equator. The nominal altitude (the “GEO altitude) of a GSO is approximately 35,786 km. An elevation angle from a user located on the Earth to a satellite in GSO is a function of the user's latitude. When a service area on the ground intended to receive communications or broadcast services (hereinafter, an “intended service area”) is at a north or south latitude above approximately 60 to 70 degrees, the elevation angle is small enough that service quality is significantly impaired.
To mitigate this problem, satellites operable in highly inclined, highly elliptical geosynchronous orbits have been proposed, as described, for example in Bigras, et al., US Pat. Pub. 2014/0017992 (hereinafter, Bigras) the disclosure of which is hereby incorporated in its entirety into the present patent application. A geosynchronous, highly inclined, elliptical orbit (HIEO) may be selected such that the orbit's apogee is located at a pre-selected, substantially constant, longitude and latitude. A satellite operating in an HIEO can, during much of its orbital period (e.g., sixteen hours out of twenty four) enable higher elevation angles to a user than a GSO satellite.
An HIEO orbit such as the one disclosed by Bigras, has an apogee altitude of about 48,000 km or higher and a perigee altitude of about 23,000 km. Where the intended service area is in the northern hemisphere, the argument of perigee (the angle in the orbital plane measured, in the direction of satellite motion, from the orbit's ascending node to the orbit perigee) for such an orbit is desirably about 270 degrees. With an argument of perigee of 270 degrees, the orbit apogee is located above the northern hemisphere and the orbit perigee is located above the southern hemisphere. Where the intended service area is in the southern hemisphere, the argument of perigee is desirably about 90 degrees. With an argument of perigee of 90 degrees, the orbit apogee is located above the southern hemisphere and the orbit perigee is located above the northern hemisphere.
Orbital debris has become a major concern in recent years. One type of orbital “debris” includes entire satellites that have been retired after the end of their operational life. To mitigate the risk that retired satellites may otherwise pose to operational satellites located in high value orbits such as GSO and low earth orbit (LEO), rules have been promulgated requiring safe disposal of satellites at end of operational life. For example, the U.S. Government Orbital Debris Mitigation Standard Practices require disposal of satellites, at end-of-life, into orbits that (i) are higher than GSO; or (ii) will result in reentry into the Earth's atmosphere within 25 years of end of operational life (EOL). Similar requirements have been incorporated into international orbital debris mitigation guidelines promulgated by the Inter-Agency Space Debris Coordination Committee (IADC) and the United Nations.
There are several known methods to accomplish satellite deorbiting. One method is to maneuver the satellite into an orbit which results in the satellite's prompt reentry into the Earth's atmosphere. This is generally impractical for a satellite initially operating in high energy orbits such as GSO and geosynchronous HIEO, because the energy required for such a maneuver is prohibitive. A second method is to place the satellite in a stable orbit above GSO altitude. This is also problematic for a satellite initially in a geosynchronous HIEO, due at least to the energy cost of raising perigee from 23,000 km to an altitude above GSO.
Thus, improved techniques for deorbiting such satellites are desirable.